GamepadButton does not retain trigger values

[bhickey]

gilrs moved LeftTrigger2 and RightTrigger2 from axes to buttons. At the time of this change the button value was stuffed into a ButtonChanged event. It appears that bevy's gamepad handling routine discards these values when constructing its just_pressed and pressed hash sets.

While some games can treat these triggers as booleans, an analog interpretation is important for configuring sensitivity or using the button as a throttle.